This invention relates to a microwave diplexer providing separate propagation paths to electromagnetic radiations of two different frequencies and, more particularly, to a diplexer having one branch constructed as a waveguide below cutoff to radiation at a lower of the two frequencies while passing radiation at a higher of the two frequencies, and having a second branch with a band pass filter for allowing propagation of the lower frequency radiation while inhibiting propagation of the higher frequency radiation.
Microwave diplexers are employed in circuits handling signals at more than one frequency band. A typical circuit is found in a communication system such as a broadcast system employing a satellite for retransmission of radio and television signals. The satellite carries an antenna with a feed structure that illuminates the antenna, or receives from the antenna, signals at different frequencies. A diplexer couples the feed structure to transceivers operating at the different frequencies.
Generally speaking, the diplexer may be described as having three branches, or channels, which are constructed of waveguide. These branches are a common branch operative at both the higher and the lower frequencies to couple electromagnetic power between the antenna and the two transceivers. The common channel branches into a through channel and a side channel which are operative to separate microwave signals at the two frequencies. The through channel connects with circuitry such as a receiver or transceiver operative at one frequency, while the side channel connects with circuitry such as a receiver or transceiver operative at the other frequency.
The diplexer is usually constructed with tuned elements in both the through and the side channels to form filters in these channels so that each of these two channels propagates radiation at only one of the two frequency bands while inhibiting propagation at the other of the frequencies. Thereby, the diplexer can separate incoming signals at the two frequency bands, and can couple outgoing signals at the two frequency bands to a common feed of the antenna.
A problem arises in that the construction of a diplexer with filters of discrete microwave elements in two of the branches adds undue complexity to the manufacturing process, and may also prevent a minimizing of the physical size of the diplexer.